All organisms possess an intrinsic ability to detect and respond to threats in their environments, but the underlying molecular mechanisms are poorly understood. A complete understanding of this process requires knowledge of the underlying neural circuits along with an ability to measure and, most importantly, perturb their activity. This is difficult to obtain in complex vertebrate circuits. However, invertebrate circuit with their well-defined neuroanatomy and quantitative behaviors are ideally placed to decipher the underlying machinery guiding complex outputs. This proposal aims to understand the neural mechanisms that code threat responses (both behavioral and physiological) in an invertebrate brain model. The nematode, Caenorhabditis elegans, provides a unique opportunity to analyze, using a multi- scale approach, genes, cells and circuits that regulate complex behaviors. The Chalasani lab has developed a novel model of threat behaviors using the interactions between C. elegans and a second nematode, Pristionchus pacificus. A starving Pristionchus will attack and devour C. elegans in 30 minutes. C. elegans in turn, will avoid both Pristionchus and its secretions. Apart from this behavioral response, C. elegans also activates mitochondrial stress upon exposure to Pristionchus. The goals of the proposed research program are to define the cellular and molecular mechanisms regulating avoidance behavior in this model system. It has already been determined that a novel neural circuit including three new sensory neurons (ASJ, ASK and ASI) drive avoidance behavior and physiological stress responses. Specific aim 1 will identify this neuronal circuit and the associated neurotransmitters and receptors that regulate predator avoidance and mitochondrial stress responses. Aim 2 will optimize an automated behavioral platform to rapidly analyze behaviors from large numbers of worms and perform a large screen for genes affecting avoidance behavior. A pilot screen has identified 4 interesting genes as required for regulating avoidance behavior. These include a TRPV channel (might be part of the Pristionchus sensing machinery), glutamate transporters and serotonin biosynthesis enzyme and serotonin re-uptake transporter. Aim 3 is focused on validating these and other candidates from the genetic screen. These studies will clarify how neural circuits process information about environmental threats at the level of synapses, neural circuits and whole organisms. Moreover, we will identify basic principles and conserved mechanisms of how neural circuits integrate glutamate and serotonin signaling to generate complex behaviors.